The determination of the positioning of a point in space and the determination of the attitude of an arbitrary object are problems relating to numerous technical fields.
The various solutions generally afforded must resolve any ambiguity in position or attitude, cater for more or less severe dynamics in terms of displacement, speeds and accelerations of the systems and satisfy high accuracy, in particular in the aeronautical field.
In systems for detecting position and attitude of objects in space catering for an accuracy of a few millimetres in position and a degree in attitude, numerous applications exist in various fields.
These systems are used in aeronautics; for detecting head posture, notably for the helmets of fighter aircraft, or of military, civilian or para-civilian helicopters. The latter case of para-civilian application may involve rescue missions at sea for example. They are also used for the detection of simulation helmets; this detection can then be combined with an oculometry device, also called an eyetracker, for detecting the position of the gaze. Numerous applications of these systems also exist in the field of virtual reality and games.
More generally, in the field of generic posture detection, there also exist numerous applications, notably in the medical field for remote operations and monitoring of instruments, in the field of position monitoring for feedback-controlled tool machines or of remote control and finally for cinema, so as to reproduce motion as synthesis images.
These various applications have technical solutions which cater for more or less constraining requirements.
Concerning applications with weak constraints, notably in terms of accuracy, there exist various systems for detecting position and/or orientation of objects.
For example, camera-based devices that recognize patches or shapes use designs printed on an object. Several cameras observe the scene and determine the spatial configuration of the observed design.
There also exist camera-based devices that recognize spheres, which are used, for example in cinema, to reconstruct human motion. The device uses several cameras which observe reflecting spheres and determine their trajectory.
Finally there exist ultrasound positioning devices relying on the principle of triangulation between ultrasonic emitters and receivers.
Concerning higher-performance applications, in particular in the aeronautical field, devices for detecting posture of helmets in aircraft use two main techniques, namely electromagnetic posture detection and electro-optical posture detection.
Electromagnetic posture detection requires devices comprising means for emitting an electromagnetic field and reception sensors on the helmet making it possible to determine their position with respect to the emitter.
Electro-optical posture detection generally requires patterns of electroluminescent diodes, also called LEDs, disposed on the helmet and several sensors of camera type mounted in the cockpit making it possible to determine the spatial configuration of a pattern of LEDs.
To improve performance, other devices comprising sensors of gyroscopic, accelero-metric or magneto-metric types are frequently combined. This hybridization of sensors makes it possible to improve the dynamic performance or to resolve an ambiguity of orientation. These sensors do not modify the static positioning performance of the detection devices cited above.
Nevertheless, these solutions exhibit a certain number of drawbacks and limitations, particularly in the aeronautical field.
Moreover, these same devices require several cameras and several sensors. The position calculations demand numerous resources and the real-time analysis is complex to implement.
Moreover, one drawback results from the diffusion in the zone of detection of the light of the LEDs and another drawback results from the disturbances of the luminous environment of the cockpit due to the sun or to stray reflections on the canopy.
As regards electromagnetic posture detection devices, robust solutions are difficult to implement.
In particular, in the aeronautical field, stray radiations and electromagnetic disturbances may degrade the performance of the existing systems.
A solution implementing a device of electro-optical type such as described in patent FR 2 905 455 makes it possible to circumvent the drawbacks of the electromagnetic devices.
Moreover, this solution preferably uses image projection means of the video-projector type.
In particular, monochromatic laser video-projectors have the advantages of emitting in a very narrow band of frequencies, a sharp image in a wide field and of making it possible to concentrate a high energy in a very small zone. The signals arising from the video-projector can very easily be distinguished from stray light.
More precisely, this solution comprises electro-optical sensors disposed on the object and distributed in groups, called clusters, analysis and calculation means making it possible to retrieve the position and/or the attitude of the object, electronic means for generating images and optical projection means comprising a display and projection optics.
The optical projection means emit in a projection cone a sharp image at every point of the displacement zone in which the object can move. Analysis of the portions of images received by the sensors of at least one cluster makes it possible to chart the position and/or the attitude of the object in the reference frame defined by the projection means, the latter consisting of a plane perpendicular to the projection axis, termed the image plane, and of the projection axis.
Finally, this solution coupled with that described in patent FR 0706132 makes it possible to define clusters whose geometric properties are notably linear sensors disposed as parallelograms, therefore coplanar, on the object, the determination of whose motion is sought.
A drawback of such a sensor is the constraint of accuracy of mechanical transfer of the sensors onto their support. Indeed, one typically seeks to obtain accuracies of the order of a milliradian in orientation on linear sensors of a length of the order of 3 cm. This imposes a transfer accuracy of the order 30 μm which must be maintained under all temperature conditions, inter alia. If the sensor is in a plane and possesses a parallelepipedal form, it must be potentially transferred onto ceramic and necessitates a very specific manufacturing process.
Moreover, this disposition contributes to the compounding of errors of mechanical tolerancing, for example in the positioning of the sensor on the helmet and as regards the accuracy of the sensor. The latter solution requires accurate calibration of the sensor, which may involve the storage of correction coefficients at sensor level so as to be able to attain the desired accuracy level.